Hyperechoes.

A novel spin-echo-based refocusing strategy called a hyperecho mechanism is introduced by which the full coherence of magnetization submitted to a sequence of arbitrary RF pulses can be reinstalled. First implementations illustrate the potential of hyperecho formation-especially for Rapid Acquisition with Relaxation Enhancement (RARE) imaging, in which the full image intensity can be retrieved using a fraction of the RF power of a fully refocused sequence. The contribution of stimulated echo pathways to the hyperecho signal leads to an increased signal intensity at a given refocusing time for tissues with T(1) > T(2). For identical T(2) contrast, longer echo times have to be used. Further possibilities for using hyperechoes in gradient-echo sequences and for spin selection are discussed. Magn Reson Med 46:6-12, 2001.